During the first hours after ischemic stroke onset, neurological deficits can be highly unstable - some patients spontaneously improve while others deteriorate. These early neurological changes are important because they have a large influence on long-term outcome. Potential mechanisms accounting for rapid improvement include fibrinolysis/ reperfusion, recruitment of collateral circulation, or endogenous neuroprotective mechanisms; while mechanisms leading to deterioration include thrombus propagation, peri-infarct spreading depression, or hemorrhagic transformation (HT). Tissue plasminogen activator (tPA), the only FDA-approved drug for the treatment of acute ischemic stroke (AIS), enhances the likelihood of fibrinolysis and reperfusion; but also increases the chances of HT. We hypothesize that genetic variant that affect pathogenic mechanism during acute ischemia may influence early neurological outcomes after AIS, and may also modulate response to IV tPA. In this grant, we propose to identify genetic variants associated with early neurological outcome in AIS patients (untreated or treated with IV tPA). These data will permit us to find novel genes/pathways and potential therapeutic targets that could improve outcome after AIS, and perhaps enhance tPA efficacy. Further, an individual's genetic profile may one day provide personalized risk stratification for treatment with IV tPA, which may guide therapeutic decisions. Currently, we have accumulated 1000 samples from phenotyped AIS patients treated with tPA from both sites (the largest such sample set in the world), which will be used for gene discovery (Discovery Series). During the grant period, we will collect an additional 3000 samples (Replication Series), which will include both tPA-treated and untreated patients so that we can determine which genetic associations are tPA-dependent or independent influences on early neurological outcomes after AIS. We propose 4 aims. Aim 1: To perform genome-wide association studies, examining early neurological improvement or deterioration after AIS. Aim 2: To determine which genetic variants that modulate levels of plasma analytes relevant to AIS pathogenesis influence early neurological outcomes. Aim 3: To replicate genome-wide associations, we will test candidate variants/genes in an independent cohort of AIS patients. And Aim 4: To determine which variants/genes associated with early outcome are tPA-dependent. At the conclusion of this grant, we will identify many variants/genes that influence early neurological outcome after AIS, and will also find variants/genes that influence outcome in a tPA-dependent manner. These findings will have both diagnostic and therapeutic implications.